1. Field of the Invention
The present invention relates to a method for carbon coating on lithium titanium oxide-based anode active material nanoparticles by calcining anode active material nanoparticles surface modified with an organic material to uniformly coat carbon on the nanoparticles, achieving high electrical conductivity leading to excellent electrochemical properties and allowing rapid transfer of lithium ions. The present invention also relates to carbon-coated lithium titanium oxide-based anode active material nanoparticles produced by the method.
2. Description of the Related Art
Lithium secondary batteries are widely used at present as power sources of information-related devices and communication devices, such as portable computers, mobile phones and cameras, due to their high energy density.
In response to recent efforts to reduce the dependence of petroleum and to reduce the emission of greenhouse gases, there has been an increasingly fierce competition to develop plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) using lithium secondary batteries as energy sources.
Furthermore, considerable research efforts have concentrated on the development of secondary batteries with the expectation that there will be a dramatically increasing demand for medium- and large-sized secondary batteries in various application fields, including robots, backup powers, medical devices, motor-driven tools and uninterruptible power supplies (UPSs).
Particularly, electric vehicles, motor-driven tools and uninterruptible power supplies require electrical charge or discharge at high rate within a short time. As power sources to meet this requirement, lithium secondary batteries are considered suitable due to their suitability for high-rate charge/discharge and good stability.
Various kinds of carbonaceous anode active materials capable of intercalating/deintercalating lithium ions are widely used at present as anode materials for lithium secondary batteries. Such carbonaceous anode active materials include artificial graphite, natural graphite, hard carbon and soft carbon.
Carbonaceous anode active materials have an operating voltage similar to that of lithium metal, are very structurally stable, and can reversibly intercalate/deintercalate lithium ions for a long time, implying excellent cycle characteristics. However, batteries using carbonaceous anode active materials have the problem of low energy density per unit volume due to low density of the carbonaceous materials. Further, since carbonaceous anode active materials have an oxidation/reduction potential by about 0.1 V lower than the oxidation/reduction potential of Li/Li+, organic electrolytes tend to decompose on the surface of carbon. In addition, the carbonaceous materials react with lithium to form solid electrolyte interface (SEI) films. The SEI films cover the surface of the carbonaceous anode active materials, causing deterioration of charge/discharge properties. Particularly, the formation of SEI films in application fields that require high-rate characteristics, such as EVs, leads to an increase in resistance upon lithium intercalation/deintercalation, and as a result, the high-rate characteristics of carbonaceous anode active materials are deteriorated. Furthermore, during charge/discharge at high rates, highly reactive lithium is deposited on the surface of anodes where it reacts with electrolytes and cathode materials. This reaction may cause safety problems, such as explosion.
Thus, there is an increasing need for novel anode active materials with high performance, safety and reliability that are suitable for the fabrication of medium- and large-sized lithium secondary batteries on an industrial scale. In recent years, lithium titanium oxides (LTOs) have received attention as anode active materials for medium- and large-sized secondary batteries because of their high performance, safety and reliability.
Since LTOs have an oxidation/reduction potential by 1.5 V higher than the potential of Li/Li+, there is little possibility that electrolyte solutions may be decomposed, which greatly decreases the possibility of SEI film formation that has been considered a problem in carbonaceous anode active materials. In addition, the high oxidation/reduction potential of LTOs reduces the possibility of deposition of metal lithium that has been considered a problem during high-rate charging/discharging of carbonaceous anode active materials, ensuring high stability during charge/discharge and enabling the utilization of LTOs in power sources of PHEVs, EVs, motor-driven tools and UPSs. In addition, the theoretical density of LTOs is about 3.5 g/cm3 and is much higher than that of carbonaceous anode active materials. Under these circumstances, LTOs are attracting a lot of attention as promising novel anode active materials of large size secondary batteries, for example, in EVs due to their high stability, excellent charge/discharge properties at high rate and high reliability.
However, the use of LTOs with a particle size at a micron level (10-100 μm; the Brunauer-Emmett-Teller (BET) specific surface area 2-5 m2/g) greatly retards the intercalation/deintercalation rate of lithium ions during charge/discharge because of their very low electrical conductivity (˜10−13 S cm−1) and very low lithium ionic conductivity. As a result, coarse LTO particles have a low charge/discharge capacity corresponding to about 70% of its theoretical charge/discharge capacity. These problems still impede the widespread utilization of LTOs as anode active materials of lithium secondary batteries.
Many approaches to increase the charge/discharge capacity of LTOs have been introduced, for example, methods for producing nano-sized LTOs to reduce the intercalation/deintercalation distance of lithium ions, methods for improving the electrical conductivity of LTOs by carbon coating or doping, and methods for producing nanostructures to increase the contact area between electrolyte solutions and electrode materials. However, the production of nano-sized particles by solid-state methods, such as ball milling, requires the consumption of high energy and additional pulverization processes, such as ball milling, for a long time, leading to poor productivity and very broad particle size distribution. On the other hand, particles produced by liquid-phase methods, such as hydrothermal methods, co-precipitation, emulsion-drying and sol-gel methods, have considerably low charge/discharge properties because of their large size and broad particle size distribution.
When nano-sized LTOs produced by ball milling are coated with carbon by solid-state methods and liquid-phase methods, their small particle size hinders uniform coating of the carbon on the surface of the particles, leading to poor charge/discharge properties.